07 September, 2008

"has a": A dog has a tail; therefore the Dog class should have a member "tail"

"is a": A poodle is a dog; therefore Poodle should either be an instance or derived class of Dog

Thinking this way really sped up my ability to design object oriented structures. Otherwise, starting out it's easy to get twisted up and start adding members to classes that should actually just be instances, or vice versa.

to which i responded:

saint_groceon's beginner advice although correct, can lead to trouble:

Objects are not just a collection of attributes and behaviors. If that was the case, dogs and cats would be indistinguishable from each other, as they both have eyes, mouths and legs, and they both eat, sleep and play. In fact, this kind of "object" is almost no different than a C struct.

Furthermore, the type of thinking described in saint_groceon's post also leads to other problems: If your "Duck" class has a "quack" method, what happens when you need to implement a rubber duck that does not quack? (you may recognize this example from Head First Design Patterns)

I agree with the fact that objects are usually nouns from the domain's language. However an object is more than that: an object is anything capable of providing a limited set of useful services. And then, with these objects "we decompose the complex world around, and assemble those objects in various ways so that they can perform useful tasks on our behalf" (David West).

West nails the definition of an objcet; the most important characteristic of an object is it's ability to be composed with other objects to create useful things, much like all of our body parts (properly assembled) make us useful creatures. So, in some respect, everything is an object!

Edit: I realize that I said little about How To Think OO, and a lot more about what OO is not. I apologize for this, but I'm no OO master, and I'm barely at the stage where I have a feeling what OO IS NOT... I'm not at the point where I can instruct on OO.

This post was mostly an exercise to help me gather my thoughts about what I've learned so far. I hope you'll find it useful nonetheless.